Light receiving element and light receiving device incorporating circuit and optical disk drive

ABSTRACT

A light receiving device includes a silicon substrate, a first P type diffusion layer on the silicon substrate, and a P type semiconductor layer on the P type diffusion layer. On a surface part of the P type semiconductor layer, two N type diffusion layers as light receiving parts, and a second P type diffusion layer between the two N type diffusion layers are provided. On the P type semiconductor layer, an antireflection film structure composed of a first silicon oxide formed by thermal oxidation and a second silicon oxide formed by CVD is provided. A film thickness of the first silicon oxide is set at about 15 nm, thus a defect in a interface between the first silicon oxide and the P type semiconductor layer is prevented. A film thickness of the second silicon oxide is set at about 100 nm, thus a leak current between cathodes is prevented when a power supply voltage is applied for long period of time.

TECHNICAL FIELD

The present invention relates to a light receiving device, a lightreceiving unit incorporating a circuit, and an optical disk drive.

BACKGROUND ART

Conventionally, an optical pickup for use in optical disk drives isstructured such that a light emitted from a semiconductor laser iscollected with a lens and radiated onto an optical disk, and the lightreflected by the optical disk with optical power modulated by a pitindicating a signal is received by a light receiving device. An electricsignal from the light receiving device is processed in a signalprocessing circuit, and a data signal written onto the optical disk isdetected, as well as a focus signal for controlling a focus of the lensand a servo signal for controlling a light collecting position on theoptical disk are detected. As the light receiving device, a so-calledsplit type light receiving device composed of a plurality of lightreceiving parts is employed for detecting the data signal, the focussignal and the servo signal.

In recent years, optical disk drives using a blue semiconductor laser asa substitute for infrared and red semiconductor lasers are beingdeveloped to support higher density data that are written onto theoptical disk. The split type light receiving device for use in suchoptical disk drive includes a conventional one shown in FIGS. 7A and 7B(Japanese unexamined patent application No. 2001-148503). FIG. 7A is aplan view showing a split type light receiving device while FIG. 7B is across sectional view taken along the arrow line D-D′ of FIG. 7A. Thesplit type light receiving device is structured such that a plurality ofN type diffusion layers 601, 601 as cathodes are provided on a P typesemiconductor layer 600 to form light receiving parts. On the surface ofthe light receiving device on the side of the light receiving parts, twofilms composed of a silicon oxide 604 and a silicon nitride 605 aredisposed to constitute an antireflection film structure 603.

The antireflection film structure 603 composed of the silicon oxide 604and the silicon nitride 605 effectively reduces the reflectance ofincident light by appropriately selecting each film thickness accordingto the wavelength of the incident light. Generally, combining aplurality of films different in kind as shown above makes it possible toobtain an antireflection film with a relatively small thickness and lowreflectance. For example, in the case of red light with wavelength of650 nm, the film thickness of the silicon oxide is set at 50 nm whilethe film thickness of the silicon nitride is set at 30 nm, so that thereflectance in the antireflection film structure 603 can reach almost4%. Further, in the case of blue light with wavelength of 400 nm, thefilm thickness of the silicon oxide is set at 10 nm while the filmthickness of the silicon nitride is set at 39 nm, so that thereflectance in the antireflection film structure 603 can reach almost0%.

Further, in the vicinity of the surface of the P type semiconductorlayer 600 and between a plurality of the light receiving parts, a P typediffusion layer 602 with impurity concentration of about 1E18 cm⁻³ to1E19 cm⁻³ is disposed so as to prevent leak current between cathodescaused by positive charges that are stored in the interface between thesilicon oxide 604 and the silicon nitride 605 and in the silicon nitride605 of the antireflection film structure 603.

However, the above-stated conventional light receiving device has aproblem that leak current between cathodes caused by positive chargesstored on the surface of the silicon nitride 605 cannot be prevented.More specifically, during a reliability test and the like after thelight receiving device is produced, if a power supply voltage is appliedto the cathode of the light receiving device for a long time, thenelectric charges present in the silicon nitride 605 of theantireflection film structure 603 are redistributed by Pool-Frenkelcurrent. Moreover, electric charges influenced by static charges andcontamination are stored on the surface of the silicon nitride 605. Leakcurrent flows between cathodes by these electric charges. FIG. 8 is agraph showing changes in leak current between cathodes as a reverse biasvoltage of the light receiving device is changed, where the horizontalaxis represents a reverse bias voltage (V) as a power supply voltageapplied to the light receiving device, and the vertical axis representscurrent (A) between cathodes. Further as shown in FIG. 9, correspondingto the length of a period of time during which the supply voltage isapplied, the leak current between cathodes is increased. In FIG. 9, thehorizontal axis represents elapsed time (hour) after the reverse biasvoltage is applied, while the vertical axis represents leak current (A)between cathodes.

The reason why the leak current flows between cathodes will be describedwith reference to schematic views shown in FIG. 10 and FIG. 11. FIG. 10is a schematic cross sectional view showing the light receiving deviceof FIG. 7B after a long-time reliability test is carried out. As shownin FIG. 10, positive charges 610 are stored on the surface of theantireflection film structure 603, and the stored positive charges 610generate inversion charges 611 in the vicinity of the surface of the Ptype semiconductor layer 600 and between the N type diffusion layers601, 601. FIGS. 11A and 11B are views showing redistribution of electriccharges caused by Pool-Frenkel current in the light receiving device ofFIG. 7B. First, as shown in FIG. 11A, during a production process of thelight receiving device, the silicon nitride 605 is damaged by plasma orthe light receiving device is formed into a chip for a wire bonding stepperformed after production of the light receiving device, by whichpositive charges 612 and negative charges 613 are generated in thesilicon nitride 605. Then, when a voltage is applied to the N typediffusion layers 601, 601 during a reliability test, the positivecharges 612 in the silicon nitride 605 are accumulated in theacross-the-width center of the silicon nitride 605 as shown in FIG. 11B,and these positive charges 612 generate inversion charges 614 in aportion of the P type semiconductor layer 600 between the N typediffusion layers 601, 601. Here, the voltage applied to the N typediffusion layers 601, 601, i.e., a reverse bias voltage of the cathode,generates a repulsive force in the silicon nitride 605, as a result ofwhich a number of positive charges 612 are concentrated in a region ofthe silicon nitride 605 corresponding to a portion between the cathodes.

As shown in FIG. 10 and FIG. 11, the generated inversion charges 611,614 are also generated in the P type diffusion layer 602 positionedbetween the N type diffusion layers 601, 601. These inversion charges611, 614 cause leak current flowing between the N type diffusion layers601, 601.

For preventing the current from flowing between the cathodes, it isnecessary to decrease an inversion voltage generated by the positivecharges, which is considered to be fulfilled by either increasing theimpurity concentration of the P type diffusion layer 602, or increasingthe thickness of the antireflection film structure 603. However, if theimpurity concentration of the P type diffusion layer 602 is increased,then carriers generated upon light reception tend to recombine,resulting in degraded sensitivity of the light receiving device. If thethickness of the silicon nitride 605 is increased for increasing thethickness of the antireflection film structure 603, stress is generatedin this silicon nitride 605, and the stress heightens an interface statebetween the P type semiconductor layer 600 and the silicon oxide 604,resulting in degradation of light receiving sensitivity. Moreover, ifthe thickness of the silicon oxide 604 is increased, the interface statebetween the P type semiconductor layer 600 and the silicon oxide 604 isheightened, causing degradation of the sensitivity of the lightreceiving device. Therefore, the film thickness of the silicon oxide 604should be about 300 nm or less while the film thickness of the siliconnitride 605 should be about 50 nm or less. However, these filmthicknesses cannot prevent leak current after application of the supplyvoltage.

Accordingly, it is a primary object of the present invention to providea light receiving device which causes almost no leak current even aftercontinual operation for a long time and which is free from degradationof sensitivity.

DISCLOSURE OF THE INVENTION

In order to achieve the above object, the present invention provides alight receiving device comprising:

-   -   a plurality of light receiving parts on a semiconductor layer;        and    -   a first light transmitting film and a second light transmitting        film which are laminated at least on the plurality of light        receiving parts and on parts between the plurality of light        receiving parts in an order from a side closer to the light        receiving parts, wherein    -   both the first light transmitting film and the second light        transmitting film are oxides, and    -   the second light transmitting film is larger in thickness than        the first light transmitting film.

According to the above configuration, both the first light transmittingfilm and the second light transmitting film are oxides, so that even ifthe thickness of the second light transmitting film is relativelylarger, a stress generated in the second light transmitting film issmaller than that in the case where the nitride oxide has a largerthickness as seen in the conventional case. Therefore, a stressgenerated in the semiconductor layer having the light receiving parts issmaller than the conventional case. As a result, the interface statebetween the first light transmitting film and the semiconductor layer issmaller than the conventional case. Further, the first lighttransmitting film is smaller in thickness than the second lighttransmitting film, so that the interface state between the first lighttransmitting film and the semiconductor layer having the light receivingparts is relatively small. Moreover, setting the total thickness of thefirst light transmitting film and the thickness of the second lighttransmitting film to be relatively large enables leak current betweenthe light receiving parts caused by electric charges stored on thesurface of the second light transmitting film to be prevented. Further,since a silicon nitride is not used in the second light transmittingfilm, redistribution of electric charges due to, for example,Pool-Frenkel current as seen in the conventional case does not occur inthe second light transmitting film, which prevents leak current betweenthe light receiving parts caused by redistribution of the electriccharges in this second light transmitting film. Thus the light receivingdevice is capable of preventing leak current between the light receivingparts, and stably achieving excellent capability free from almost anydegradation of sensitivity.

Here, for example, by setting the thickness of the first lighttransmitting film and the second light transmitting film atλ/4N(2M+1)nm, where λ(nm) is wavelength of incident light, N is an indexof refraction of each of the first and second light transmitting films,and M is an integer, degradation of the sensitivity of the lightreceiving device may be effectively prevented.

Here, the second light transmitting film may be disposed directly orindirectly on the first light transmitting film.

In one embodiment, the first light transmitting film is a silicon oxideformed by thermal oxidation method, and

-   -   the second light transmitting film is a silicon oxide formed by        deposition method.

According to the above embodiment, the first light transmitting filmwith a relatively small thickness is formed by thermal oxidation method,which makes it possible to reduce defect generated on the interfacebetween the first light transmitting film and the semiconductor layerhaving the light receiving parts. This makes it possible to preventdegradation of the sensitivity of the light receiving device caused bythe defect of the interface. Further, the second light transmitting filmwith a relatively large thickness is formed by deposition method, whichmakes it possible to make a stress generated between the second lighttransmitting film and the first light transmitting film relativelysmall. Therefore, in the interface between the first light transmittingfilm and the semiconductor layer, the interface state caused by thestress may be lowered. This makes it possible to obtain the lightreceiving device with smaller leakage between cathodes as seen in theconventional case and with excellent sensitivity.

Here, the deposition method refers to CVD (Chemical Vapor Deposition),PVD (Physical Vapor Deposition), liquid-phase growth, vapor deposition,and sputtering.

In one embodiment, the light receiving device further comprises

-   -   a third light transmitting film disposed between the first light        transmitting film and the second light transmitting film.

According to the above embodiment, the third light transmitting film isdisposed between the first light transmitting film and the second lighttransmitting film, which makes it possible to reduce the stress by thesecond light transmitting film. Therefore, the interface state caused bythe stress on the interface can be lowered, and so the sensitivity ofthe light receiving device in the present embodiment can be furtherincreased.

In one embodiment, the third light transmitting film is a siliconnitride.

According to the above embodiment, the third light transmitting film isa silicon nitride, so that the stress by the second light transmittingfilm is effectively reduced, and the sensitivity of the light receivingdevice in the present invention is effectively increased. Further, thesilicon nitride that is the third light transmitting film is coveredwith the second light transmitting film, so that the silicon nitride isnot exposed during the production process or in the wire bonding step.Therefore, since almost no electric charges are stored in the siliconnitride, leak current between the light receiving parts caused byredistribution of electric charges due to, for example, Pool-Frenkelcurrent is effectively prevented.

Furthermore, a light receiving unit incorporating a circuit of thepresent invention comprises:

-   -   the aforementioned light receiving device; and    -   a signal processing circuit for processing a signal from light        receiving parts of the light receiving device, wherein    -   the light receiving device and the signal processing circuit are        formed on the semiconductor layer.

According to the above configuration, the light receiving device and thesignal processing circuit are formed in a monolithic state, which makesit possible to provide a small-size light receiving unit incorporating acircuit with smaller leak current as well as excellent sensitivity.

Furthermore, an optical disk drive of the present invention comprisesthe aforementioned light receiving device or the aforementioned lightreceiving unit incorporating a circuit.

According to the above configuration, the small-size light receivingdevice or the small-size light receiving unit incorporating a circuitwith smaller leak current as well as excellent sensitivity is provided,which makes it possible to provide an optical disk drive with stableoperation capable of, for example, reading and writing mass-storage dataat high speed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view showing a light receiving device in a firstembodiment of the present invention, while FIG. 1B is a cross sectionalview taken along the arrow line A-A′ of FIG. 1A;

FIG. 2 is a graph showing leak current flowing between cathodes when apower supply voltage is applied for 1000 hours to the light receivingdevice in the first embodiment;

FIG. 3A is a plan view showing a light receiving device in a secondembodiment of the present invention, while FIG. 3B is a cross sectionalview taken along the arrow line B-B′ of FIG. 3A;

FIG. 4A is a plan view showing a light receiving device in a thirdembodiment of the present invention, while FIG. 4B is a cross sectionalview taken along the arrow line C-C′ of FIG. 4A;

FIG. 5 is a cross sectional view showing a light receiving unitincorporating a circuit in a fifth embodiment of the present invention;

FIG. 6 is a view showing an optical pickup in an optical disk drive in asixth embodiment of the present invention;

FIG. 7A is a plan view showing a conventional light receiving device,while FIG. 7B is a cross sectional view taken along the arrow line D-D′of FIG. 7A;

FIG. 8 is a graph showing changes in leak current between cathodes as areverse bias voltage of the light receiving device is changed;

FIG. 9 is a graph showing changes in leak current between cathodescorresponding to the length of application time of a power supplyvoltage;

FIG. 10 is a schematic cross sectional view showing a conventional lightreceiving device after a longtime reliable test is carried out;

FIGS. 11A and 11B are views showing the conventional light receivingdevice with electric charges caused by Pool-Frenkel current beingrecombined during a reliability test, where FIG. 11A is a crosssectional view showing the state before the reliability test, while FIG.11B is a cross sectional view showing the state after the reliabilitytest.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the invention will now be described in details withreference to the accompanying drawings.

THE FIRST EMBODIMENT

FIG. 1A and 1B are views of a light receiving device in the firstembodiment of the present invention. FIG. 1A is a plan view showing thelight receiving device while FIG. 1B is a cross sectional view takenalong the arrow line A-A′ of FIG. 1A. This light receiving device is asplit type light receiving device having a plurality of light receivingparts. In this embodiment, contacts, metal interconnections, interlayerinsulating films and the like that are formed after a contact step aredeleted.

The light receiving device includes a silicon substrate 100, a first Ptype diffusion layer 101 with impurity concentration of about 1E18 cm⁻³and a thickness of about lam on the silicon substrate 100, and a P typesemiconductor layer 102 with impurity concentration of about 1E13 to1E16 cm⁻³ and a thickness of about 10 μm to 20 μm on the P typediffusion layer 101. On the surface part of the P type semiconductorlayer 102, two N type diffusion layers 103 and 103 with impurityconcentration of about 1E17 to 1E20 cm⁻³ in the vicinity of the surfaceand a junction depth of about 0.2 μm to 1.5 μm are formed to constitutea plurality of light receiving parts. An impurity forming the N typediffusion layers 103 may be any element such as arsenic, phosphorus, andantimony as long as it is pentavalent. It is to be noted that the numberof the N type diffusion layers 103 may be two or more.

In the surface part of the P type semiconductor layer 102 and betweenthe two N type diffusion layers 103 and 103, a second P type diffusionlayer 104 is provided. In the second P type diffusion layer 104, thereis diffused an impurity with such concentration that allows restrain ofleak current between a plurality of the N type diffusion layers 103 whena power supply voltage is continuously applied for a long time, andallows sufficient securement of the sensitivity of the light receivingparts. More specifically, the second P type diffusion layer 104 hasimpurity concentration of about 1E17 cm⁻³. Further, on the bothleft-hand and right-hand side of the P type semiconductor layer 102 inFIG. 1B, there are shown parts of a third P type diffusion layer 105extending from the top surface of the P type semiconductor layer 102 tothe P type diffusion layer 101 for taking a contact between the topsurface of the P type semiconductor layer 102 and the P type diffusionlayer 101. It is to be noted that impurities forming the first to thethird P type diffusion layers 101, 104 and 105 may be any elements suchas boron and indium as long as the elements are triatomic.

Further, on the P type semiconductor layer 102, and on the N typediffusion layers 103 as well as on a part between these two N typediffusion layers 103, 103, there is provided an antireflection filmstructure 106. The antireflection film structure 106 is composed of afirst silicon oxide 107 and a second silicon oxide 108 laminated in theorder from the side closer to the light receiving parts. The firstsilicon oxide 107 is an oxide formed by thermal oxidation, whereas thesecond silicon oxide 108 is an oxide formed by CVD. The oxide formed byCVD is lower in density than the oxide formed by thermal oxidation,whereas the oxide formed by CVD is larger in etching rate than the oxideformed by thermal oxidation. The total film thickness of the firstsilicon oxide 107 and the second silicon oxide 108, i.e., the thicknessof the antireflection film structure 106, should be set such that aninversion voltage specified by the total thickness and the impurityconcentration of the second P type diffusion layer 104 is not smallerthan the power supply voltage of the light receiving device. Theinversion voltage is expressed in the following formula under an idealcondition;V_(th) ={square root}{square root over (2 ε _(si) ε ⁰ q N _(a) (2 Ø _(b)))}/ε _(ox) ε₀/t_(ox))+2 Ø_(b)   (1)where “ε_(si)” denotes a relative permittivity of the silicon, “ε₀”denotes a space permittivity, “q” is an elementary charge quantity,“N_(a)” denotes an impurity concentration on the surface of the layerwith an antireflection film structure formed on the surface, “Ø_(b)”denotes a difference between Fermi level and intrinsic Fermi level inthe layer with an antireflection film structure formed on the surface,“ε_(ox)” denotes a relative permittivity of the antireflection filmstructure, and “t_(ox)” is the thickness of the antireflection filmstructure.

In the above formula (1), setting the inversion voltage V_(th) at notsmaller than 6V makes it possible to prevent inversion of electriccharges in a region directly under the antireflection film structure106. In the case where the impurity concentration N_(a) in the aboveformula (1) is set to be the impurity concentration of the P typediffusion layer 104 in the present embodiment, it is necessary to sett_(ox) of the thickness of the antireflection film structure 106 at 110nm to 120 nm or larger. Further, in order to prevent defect from beinggenerated in the interface between the P type semiconductor layer 102and the first silicon oxide 107 formed by thermal oxidation, it isnecessary to set the film thickness of the first silicon oxide 107 atabout 30 nm or lower. Therefore, it is necessary to form the firstsilicon oxide 107 thinner than the second silicon oxide 108. In thepresent embodiment, the film thickness of the first silicon oxide 107 isset at about 15 nm, while the film thickness of the second silicon oxide108 is set at about 100 nm. In this case, a value of inversion voltagespecified by the impurity concentration of the second P type diffusionlayer 104 and the thickness of the antireflection film structure 106 isabout 6.5 V or larger. Therefore, even if a power supply voltage of 6Vis continuously applied for a long time, leak current caused by theinversion charges stored on the surface part of the second P typediffusion layer 104 does not flow between the two N type diffusionlayers 103. FIG. 2 is a view showing the result of measurement of leakcurrent flowing between the two N type diffusion layers 103, i.e.,between the cathodes of the light receiving device, when a power supplyvoltage is applied to the light receiving device of FIGS. 1A and 1B for1000 hours. In FIG. 2, the horizontal axis represents elapsed time(hour) after the power supply voltage is applied, while the verticalaxis represents leak current (A) between the cathodes. As shown in FIG.2, in the light receiving device, leak current hardly flows between thecathodes even after the power supply voltage is applied for 100 hours.Further, since the film thickness of the first silicon oxide 107 is setat about 30 nm or smaller, defect is not generated in the interfacebetween the P type semiconductor layer 102 and the first silicon oxide107 and therefore the sensitivity of the light receiving device is notdegraded due to the interface state caused by defect. Therefore, thelight receiving device in the present embodiment makes it possible tostably reduce leak current and to achieve excellent sensitivity.

In the above embodiment, the thickness of the first silicon oxide 107and the second silicon oxide 108 constituting the antireflection filmstructure 106 should be such that the first silicon oxide 107 is thinnerthan the second silicon oxide 108 and defect is not generated in theinterface between the first silicon oxide 107 and the P typesemiconductor layer 102. The impurity concentration of the second P typediffusion layer 104 should be such that the inversion voltage is largerthan the power supply voltage under the thickness of the antireflectionfilm structure 106.

Further, the present invention is not limited to the configuration ofthe split type light receiving device disclosed in the above embodiment,and so is applicable to light receiving devices with variousconfigurations. For example, as long as leak current between cathodesimmediately after production or during a reliability test issufficiently restrained, the P type diffusion layer 104 may have otherconcentration, or the P type diffusion layer 104 may be deleted.

Further, in the above embodiment, it is acceptable to change P type to Ntype and N type to P type in the configuration. Moreover, the form ofthe light receiving parts is not limited to the form shown in FIG. 1Aand therefore other forms are applicable. Also, the silicon substrate100 may be other semiconductor substrates. Furthermore, the power supplyvoltage of the light receiving device is not limited to the voltagevalue disclosed in the present embodiment.

Further in the above embodiment, although the second silicon oxide 108is formed by CVD, it may be formed by any other methods as PVD,liquid-phase growth, vapor deposition and sputtering, except the thermaloxidation.

THE SECOND EMBODIMENT

FIG. 3A is a plan view showing a light receiving device in the secondembodiment of the present invention, while FIG. 3B is a cross sectionalview taken along the arrow line B-B′ of FIG. 3A. In this embodiment,contacts, metal interconnections, interlayer insulating films and thelike that are formed after a contact step are deleted.

The light receiving device of the present embodiment includes a siliconsubstrate 200, a first P type diffusion layer 201 with impurityconcentration of about 1E18 cm⁻³ and a thickness of about 1 μm on thesilicon substrate 200, and a P type semiconductor layer 202 withimpurity concentration of about 1E13 to 1E16 cm⁻³ and a thickness ofabout 10 μm to 20 μm. On the surface part of the P type semiconductorlayer 202, two N type diffusion layers 203 and 203 with impurityconcentration of about 1E17 to 1E20 cm⁻³ in the vicinity of the topsurface and a junction depth of about 0.2 μm to 1.5 μm are formed toconstitute two light receiving parts. The light receiving device is aso-called split type light receiving device having a plurality of lightreceiving parts. An impurity forming the N type diffusion layers 203 maybe any element such as arsenic, phosphorus, and antimony as long as theelement is pentavalent. It is to be noted that the number of the N typediffusion layers 203 may be two or more.

In the surface part of the P type semiconductor layer 202 and betweenthe two N type diffusion layers 203 and 203, a second P type diffusionlayer 204 is provided, and the second P type diffusion layer 204 hasimpurity concentration of about 1E17 cm⁻³ like the first embodiment.Further, on the both left-hand and right-hand sides of the P typesemiconductor layer 202 in FIG. 3B, there are shown parts of a third Ptype diffusion layer 205 from the top surface of the P typesemiconductor layer 202 to the first P type diffusion layer 201 fortaking a contact. It is to be noted that impurities forming the first tothe third P type diffusion layers 201, 204 and 205 may be any elementssuch as boron and indium as long as the elements are triatomic.

Further, on the P type semiconductor layer 202, and on the N typediffusion layers 203 as well as on a part between the two N typediffusion layers 203 and 203, there is provided an antireflection filmstructure 206. The antireflection film structure 206 is composed of afirst silicon oxide 207, a second silicon oxide 208, and a siliconnitride 209, laminated in the order from the side closer to the lightreceiving parts. The first silicon oxide 207 is an oxide formed bythermal oxidation, whereas the second silicon oxide 208 is an oxideformed by CVD. The entire thickness of the antireflection film structure206 should be set such that an inversion voltage specified by the entirethickness and the impurity concentration of the second P type diffusionlayer 204 is not smaller than the power supply voltage of the lightreceiving device. More specifically, in the case where the power supplyvoltage is 6V, it is necessary to set the entire thickness of theantireflection film structure 206 at not smaller than 110 nm. Herein, toprevent defect from being generated in the interface between the firstsilicon oxide 207 formed by thermal oxidation and the P typesemiconductor layer 202, the film thickness of the first silicon oxide207 is set at not larger than 30 nm. Also, forming the silicon nitride209 with a larger thickness increases the stress of the second siliconoxide 208, and therefore the thickness of the silicon nitride 209 shouldpreferably be set at as small as possible, specifically at not largerthan 50 nm. Further, it is necessary to control reflectance of theantireflection film structure 206 by regulating the film thickness ofeach of the first silicon oxide 207, the second silicon oxide 208 andthe silicon nitride 209 corresponding to the wavelength of incidentlight. Based on the above fact, the thickness of the first silicon oxide207 is set at about 16 nm, the thickness of the second silicon oxide 208is set at about 150 nm, and the thickness of the silicon nitride 209 isset at about 50 nm. With this thickness of the antireflection filmstructure 206 and the impurity concentration of the second P typediffusion layer 204, the inversion voltage is about 7.7V. Therefore,even if positive charges are stored on the surface of the siliconnitride 209, inversion charges are not stored in a part of the P typesemiconductor layer 202 between the two N type semiconductor layers 203during a reliability test with a power supply voltage of about 6V.Further, even if carriers in the silicon nitride 209 is redistributed byPool-Frenkel current, an inversion voltage specified by the totalthickness of the first silicon oxide 207 and the second silicon oxide208 and the impurity concentration of the second P type diffusion layer204 is about 6.2V, so that the inversion charges are not stored in thepart of the P type semiconductor layer 202 between the two N typesemiconductor layers 203 during the reliability test with a power supplyvoltage of about 6V. Therefore, leak current is not generated betweenthe N type semiconductor layers 203, i.e., between the cathodes. Also,the silicon nitride 209 is provided on the second silicon oxide 208,which makes it possible to reduce the reflectance of the antireflectionfilm structure 206 to several %, thereby the split type light receivingdevice having excellent characteristics and sensitivity is obtained.

In the above embodiment, as long as leak current between cathodesimmediately after production or during a reliability test issufficiently restrained, the second P type diffusion layer 204 may haveother concentration, or the second P type diffusion layer 204 may bedeleted.

Further, in the above embodiment, it is acceptable to change P type to Ntype and N type to P type in the configuration. Moreover, the lightreceiving parts may adopt other forms. Furthermore, the power supplyvoltage of the light receiving device is not limited to the voltagevalue disclosed in the present embodiment.

Further in the above embodiment, although the second silicon oxide 208is formed by CVD, it may be formed by any other methods as PVD,liquid-phase growth, vapor deposition and sputtering, except the thermaloxidation.

THE THIRD EMBODIMENT

FIG. 4A is a plan view showing a light receiving device in a thirdembodiment of the present invention, while FIG. 4B is a cross sectionalview taken along the arrow line C-C′ of FIG. 4A.

The light receiving device includes a silicon substrate 300, a first Ptype diffusion layer 301 with impurity concentration of about 1E18 cm⁻³and a thickness of about 1 μm on the silicon substrate 300, and a P typesemiconductor layer 302 with impurity concentration of about 1E13 to1E16 cm⁻³ and a thickness of about 10 μm to 20 μm on the first P typediffusion layer 301. On the surface part of the P type semiconductorlayer 302, two N type diffusion layers 303 and 303 with impurityconcentration of about 1E17 to 1E20 cm⁻³ in the vicinity of the topsurface are formed to constitute light receiving parts. An impurityforming the N type diffusion layers 303 may be any element such asarsenic, phosphorus, and antimony as long as the element is pentavalent.It is to be noted that the number of the N type diffusion layers 303 maybe two or more.

In the surface part of the P type semiconductor layer 302 and betweenthe two N type diffusion layers 303 and 303, a second P type diffusionlayer 304 is provided. Like the first and second embodiments, the secondP type diffusion layer 304 has impurity concentration of about 1E17cm⁻³. Further, on the both left-hand and right-hand sides of the P typesemiconductor layer 302 in FIG. 4B, there are shown parts of a third Ptype diffusion layer 305 extending from the top surface of the P typesemiconductor layer 302 to the P type diffusion layer 301 for taking acontact. It is to be noted that impurities forming the first to thethird P type diffusion layers 301, 304 and 305 may be any elements suchas boron and indium as long as the elements are triatomic.

Further, on the P type semiconductor layer 302, and on the N typediffusion layers 303 as well as on a part between these two N typediffusion layers 303 and 303, there is provided an antireflection filmstructure 306. The antireflection film structure 306 is composed of afirst silicon oxide 307, a silicon nitride 308, and a second siliconoxide 309, laminated in the order from the side closer to the lightreceiving parts. The first silicon oxide 307 is formed by thermaloxidation, whereas the second silicon oxide 309 is formed by CVD.

The total thickness of the antireflection film structure 306 should beset such that an inversion voltage specified by the total thickness andthe impurity concentration of the second P type diffusion layer 304 isnot smaller than the power supply voltage of the light receiving device.In the case where the power supply voltage is 6V, it is necessary to setthe total film thickness at about 120 nm or larger. Particularly, thefirst silicon oxide 306 should preferably be as thin as possible, andthe thickness thereof should preferably be not larger than 30 nm. Also,the silicon nitride 308 with a larger thickness increases the stress andtherefore the thickness thereof should preferably be reduced to about 50nm. Therefore, it is necessary to form the first silicon oxide 307thinner than the second silicon oxide 309. More particularly, the filmthickness of the first silicon oxide 307 is set at about 10 nm to 20 nm,the thickness of the silicon nitride 308 is set at about 10 nm to 30 nm,and the thickness of the second silicon oxide 309 is set at about 40 nmto 50 nm. In this case, an inversion voltage specified by impurityconcentration of the P type semiconductor layer 302 and the totalthickness of the antireflection film structure 306 is about 6 to 7V orlarger. This makes it possible to effectively restrain leak currentbetween cathodes conventionally generated after a power supply voltageis continuously applied for a long time. Also, the silicon nitride 308is provided between the first silicon oxide 307 formed by thermaloxidation and the second silicon oxide 309 formed by CVD other thanthermal oxidation, which makes it possible to alleviate the stress bythe second silicon oxide 309 formed by CVD. Also, in the configurationdisclosed in this embodiment, the second silicon oxide 309 is formed onthe silicon nitride 308, so that almost no electric charges are storedin the silicon nitride 308 during the production process and in the wirebonding step. This allows effective prevention of leak current betweenlight receiving parts caused by redistribution of electric chargesgenerated in the silicon nitride during operation of the light receivingdevice as seen in the conventional case.

In the above embodiment, the thickness of the films constituting theantireflection film structure 306 may take various combinations as longas the inversion voltage is not smaller than a power supply voltage andthe film thickness of the first silicon oxide 307 is not larger than 30nm.

Further, the second silicon oxide 309 may be formed by any other methodsas PVD, liquid-phase growth, vapor deposition and sputtering than CVDexcept the thermal oxidation.

THE FOURTH EMBODIMENT

A light receiving device in the fourth embodiment of the presentinvention has the same components as the light receiving device in thethird embodiment shown in FIGS. 4A and 4B except that the thickness ofthe antireflection film structure is different from that of the lightreceiving device in the third embodiment. In this embodiment,description will be given with reference numerals used in the lightreceiving device in the third embodiment.

In the light receiving device in this embodiment, each film constitutingthe antireflection film structure 306 is formed to have a thicknesscorresponding to the wavelength of light that comes incident to thelight receiving device. More specifically, when the wavelength of theincident light to the light receiving device is 400 nm and the powersupply voltage is 6V, the first silicon oxide 307 is formed to have athickness of 16 nm, the silicon nitride 308 is formed to have athickness of about 30 nm, and the second silicon oxide 308 is formed tohave a thickness of about 140 nm. This makes it possible to reduce thereflectance of the entire antireflection film structure 306 to severalpercent. Also, an inversion voltage specified by each thickness of filmconstituting the antireflection film structure 306 and the impurityconcentration of the second P type diffusion layer 304 is about 9V.Therefore, the light receiving device has an inversion voltage notsmaller than the power supply voltage of 6V, so that even if the powersupply voltage is continuously applied for a long time, leak currenthardly flows between the cathodes. Therefore, the light receiving deviceof the present embodiment has a good signal characteristic that a powerof incident light is efficiently converted to a signal, and is capableof preventing leak current for a long time and stably maintaining goodcharacteristics.

THE FIFTH EMBODIMENT

FIG. 5 is a cross sectional view showing a light receiving unitincorporating a circuit in a fifth embodiment of the present invention.The light receiving unit incorporating a circuit is composed of a lightreceiving device D having the same components as the light receivingdevice in the fourth embodiment and a bipolar transistor T as a signalprocessing circuit for processing a signal from the light receivingdevice D formed on the same semiconductor layer. In this embodiment,description of multilevel interconnections and interlayer films formedafter the step for processing metal interconnections is deleted.

The light receiving unit incorporating a circuit has a silicon substrate400 and a first P type diffusion layer 401 on the silicon substrate 400.The silicon substrate 400 has boron concentration of about 1E15 cm⁻³,meanwhile the first P type diffusion layer 401 has a thickness of 1 to 2μm and boron concentration of about 1E18 to 1E19 cm⁻³ for decreasingparasitic resistance against the anode of the light receiving device D.On the first P type diffusion layer 401, there is formed a first P typesemiconductor layer 402 with thickness of 15 to 16 μm and boronconcentration of about 1E13 to 1E14 cm⁻³.

On the first P type semiconductor layer 402, there is formed a secondsemiconductor layer 403 with a thickness of 1 to 2 μm and boronconcentration of about 1E13 to 1E14 cm³. LOCOS oxides 404, 404, . . .are formed on the second P type semiconductor layer 403 for separatingthe element.

Further, on the surface part of the second P type semiconductor layer403, there are formed two first N type diffusion layers 405 and 405 withphosphorus concentration of 1E19 to 1E20 cm⁻³ and a junction depth ofabout 0.2 to 1.5 μm to form two light receiving parts. The N typediffusion layers 405 may be formed with use of pentavalent elements suchas arsenic and antimony instead of phosphorus.

In the surface part of the P type semiconductor layer 402 and betweenthese two first N type diffusion layers 405 and 405, a second P typediffusion layer 406 with impurity concentration of about 1E17 cm⁻³ isprovided.

Further, on the second P type semiconductor layer 403, and on lightreceiving parts as well as on a part between these light receivingparts, there is provided an antireflection film structure 407 composedof a plurality of light transmitting films. Like the fourth embodiment,the antireflection film structure 407 is composed of a first siliconoxide 408 with a film thickness of 16 nm, a silicon nitride 409 with afilm thickness of 30 nm, and a second silicon oxide 410 with a filmthickness of 140 nm, laminated in the order from the side closer to thelight receiving parts. The second silicon oxide 410 may be formed on thesurface of the transistor T for serving as an interlayer film and/or acover film for protecting the transistor.

Further, for forming an interconnection with the first P type diffusionlayer 401 on the top surface of the second P type semiconductor layer403, a third P type diffusion layer 411 with boron concentration ofabout 1E18 to 1E19 cm⁻³ is formed so as to extend from the top surfaceof the second P type semiconductor layer 403 to the first P typediffusion layer 401.

Further, in a region of the second P type semiconductor layer 403 forforming the transistor T, an N type well structure 412 with phosphorusconcentration of 1E17 to 1E19 cm⁻³ is formed. Under the N type wellstructure 412, there is provided a second N type diffusion layer 413with phosphorus concentration of about 1E18 to 1E19 cm⁻³ for decreasingresistance of the N type well structure 412.

In part of the region of the N type well structure 412, there is formeda first N type semiconductor layer 414 with a phosphorus concentrationof 1E19 to 2E19 cm⁻³ as a collector contact of the transistor T. Inother part of the region of the N type well structure 412, there areformed a third P type semiconductor layer 415 with boron concentrationof 1E17 to 1E19 cm⁻³ as a base of the transistor, and a second N typesemiconductor layer 416 as an emitter formed with arsenic.

Further, there are formed a cathode electrode (unshown) connected to thefirst N type diffusion layers 405 and an anode electrode 417 connectedto the third P type diffusion layer 411 of the light receiving device D.Further, there are formed a collector electrode 418, a base electrode419 and an emitter electrode 420 of the transistor T. In theabove-configured light receiving unit incorporating a circuit, the lightreceiving device D has the antireflection film structure 407 composed ofthe first silicon oxide 408, the silicon nitride 409 and the secondsilicon oxide 410. Further, the light receiving device D and thetransistor T are formed on the same semiconductor substrate. Therefore,such a light receiving unit is obtained as small-size, having excellentsensitivity and signal characteristic, efficiently converting a power ofincident light to a signal, and having a stable capability with leakcurrent being prevented for a long time.

Further, the light receiving device D may be a light receiving deviceother than that in the fourth embodiment.

In the above embodiment, although the bipolar transistor T is an NPNtransistor, it may be a PNP transistor or combination of the NPN typeand PNP type transistors. Furthermore, without being limited to thebipolar transistor, the transistor may be other transistor such as MOS(Metal-Oxide-Semiconductor) transistor or BiCMOS (Bipolar CMOS)transistor, and further the transistor may be replaced by other signalprocessing circuit.

THE SIXTH EMBODIMENT

FIG. 6 is a view showing an optical pickup in an optical disk drive in asixth embodiment of the present invention. The optical pickup includesthe light receiving device 506 of the present invention having fivelight receiving parts from D1 to D5.

The optical pickup splits light from a semiconductor laser 500 intothree beams, consisting of two side beams for tracking and one main beamfor reading signals, with use of a diffraction grating 501 forgenerating tracking beams. These three beams are transmitted through ahologram element 502 as zero-order light, converted to parallel beams bya collimate lens 503, and collected on a disk 505 by an object lens 504.

The collected light is reflected with a light power being modulated by apit formed on the disk 505. The reflected light transmits through theobject lens 504 and the collimate lens 503, and is diffracted by thehologram element 502. A primary light from the hologram element 502comes incident to a light receiving device 506 having five lightreceiving parts from D1 to D5. Then, by adding to and subtracting fromeach other among outputs from these five light receiving parts, a datasignal and a tracking signal are obtained.

An optical disk drive having the above-configured optical pickupincludes the light receiving device 506 capable of preventing leakcurrent for a long time and having excellent sensitivity and signalcharacteristic. Therefore, it is attained that the optical disk drive issuitable for an optical disk for high-density storage such as DVD withuse of light having short wavelength such as blue light.

In the above embodiment, the light receiving device 506 may be a lightreceiving unit incorporating a circuit of the present invention. Thismakes it possible to form a light receiving device and a circuit forprocessing a signal from the light receiving device on one chip,resulting in a small-size optical pickup and thereby a small-sizeoptical disk drive.

Further, without being limited to the optical pickup system in the aboveembodiment, the present invention may use other optical systems.

1. A light receiving device comprising: a plurality of light receivingparts on a semiconductor layers; and a first light transmitting film anda second light transmitting film which are laminated at least on theplurality of light receiving parts and on parts between the plurality oflight receiving parts in an order from a side closer to the lightreceiving parts, wherein both the first light transmitting film and thesecond light transmitting film are oxides, and the second lighttransmitting film is larger in thickness than the first lighttransmitting film.
 2. The light receiving device as defined in claim 1,wherein the first light transmitting film is a silicon oxide formed bythermal oxidation method, and the second light transmitting film is asilicon oxide formed by deposition method.
 3. The light receiving deviceas defined in claim 1, further comprising a third light transmittingfilm disposed between the first light transmitting film and the secondlight transmitting film.
 4. The light receiving device as defined inclaim 3, wherein the third light transmitting film is a silicon nitride.5. A light receiving unit incorporating a circuit comprising: the lightreceiving device as defined in claim 1; and a signal processing circuitfor processing a signal from light receiving parts of the lightreceiving device, wherein the light receiving device and the signalprocessing circuit are formed on the semiconductor layer.
 6. An opticaldisk drive comprising the light receiving device as defined in claim
 17. An optical disk drive comprising the light receiving unitincorporating a circuit as defined in claim
 5. 8. A light receivingdevice comprising: a plurality of light receiving parts on asemiconductor layer; and a first light transmitting film and a secondlight transmitting film which are laminated at least on the plurality oflight receiving parts and on parts between the plurality of lightreceiving parts in an order from a side closer to the light receivingparts, wherein both the first light transmitting film and the secondlight transmitting film are oxides, a material of the first lighttransmitting film and a material of the second light transmitting filmare different from each other, and the second light transmitting film islarger in thickness than the first light transmitting film.